Light-emitting device including a plurality of organic electroluminescent elements

ABSTRACT

A light-emitting device includes a plurality of organic EL elements. Each of the organic EL elements includes a reflection electrode, a hole transport region, an electron-trapping luminescent layer, and a light extraction electrode in this order. The hole transport region has a sheet resistance of 4.0×10 7  Ω/sq.⋅ or more at a current of 0.1 nA/pixel, and the total thickness of the hole transport region and the electron-trapping luminescent layer is equivalent to an optical path length enabling emission from the electron-trapping luminescent layer to be enhanced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation, and claims the benefit, of U.S.patent application Ser. No. 16/162,203, filed on Oct. 16, 2018, andclaims the benefit of Japanese Patent Application No. 2017-202820 filedOct. 19, 2017 and No. 2018-164460 filed Sep. 3, 2018, which are herebyincorporated by reference herein in their entirety.

BACKGROUND Field of the Disclosure

The present disclosure relates to a light-emitting device including aplurality of organic EL elements and to an image forming device, adisplay device, and an imaging apparatus.

Description of the Related Art

An organic electroluminescent element (hereinafter referred to as anorganic EL element) includes a pair of electrodes and an organiccompound layer between the pair of electrodes. In a known element, thepair of electrodes consist of a metal electrode including a metalreflection layer and a transparent electrode. Organic EL elementsoperable at a low voltage have recently attracted attention. Suchorganic EL elements have advantageous features including surfaceemission, low weight, and good visibility and are being used in practiceas light-emitting devices in flat displays, lighting devices,head-mounted displays, and print head light sources ofelectrophotographic printers.

In particular, there is an increasing demand for high-definition organicEL display devices, and a type using white organic EL elements and colorfilters (hereinafter referred to as a white+CF type) is known. Thewhite+CF type can be manufactured with a higher yield than a type usinga very fine metal mask because the organic compound layer of thewhite+CF type is formed over the entire surface of the substrate byvapor deposition. Also, since the pixel size and the interval betweenthe pixels are not restricted by the deposition precision of the organiccompound layer, the definition of white+CF organic EL display devices isincreased relatively easily.

However, when the organic compound layer is shared by all the organic ELelements, leakage of the driving current is likely to occur between anytwo adjacent organic EL elements. Consequently, the pixels that shouldnot emit light are affected by the pixels that should emit light,thereby emitting a slight amount of unwanted light. This causes adecrease in color gamut and efficiency.

Japanese Patent Laid-Open No. 2012-216338 (hereinafter referred to asPTL 1) discloses that the thickness of the portion of the organiccompound layer between the pixels is reduced by forming a groove in theinsulating layer between the pixels. Since the resistance of the thinnedportion of the organic compound layer is increased, leakage current isreduced.

Japanese Patent Laid-Open No. 2014-232631 (hereinafter referred to asPTL 2) discloses a structure having a discontinuous organic compoundlayer formed by reversely tapering the ends of insulating partitionsbetween the pixels. The discontinuous organic compound layer reducesleakage current.

In the structures disclosed in PTL 1 and PTL 2, however, the steep formthereof may cause the sealing layer to be degraded or the upperelectrode to be disconnected.

It is known that by reducing the thickness of the organic compoundlayer, the resistance is increased. However, when the reflectionelectrode and the luminescent layer are close to each other, opticalinterference does not occur effectively and, accordingly, the opticalelement requires high power consumption.

SUMMARY

Accordingly, the present disclosure provides a light-emitting device inwhich leakage current between neighboring organic EL elements is reducedand in which optical interference occurs effectively, thereby reducingthe power consumption.

According to an aspect of the present disclosure, there is provided alight-emitting device including a plurality of organic EL elements. Eachof the organic EL elements includes a reflection electrode, a holetransport region, an electron-trapping luminescent layer, and a lightextraction electrode in this order. The hole transport region has asheet resistance of 4.0×10⁷ Ω/sq. or more at a current of 0.1 nA/pixel,and the total thickness of the hole transport region and theelectron-trapping luminescent layer is equivalent to an optical pathlength enabling emission from the electron-trapping luminescent layer tobe enhanced.

Further features of the present disclosure will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a light-emitting deviceaccording to an embodiment of the present disclosure.

FIG. 2 is a plot showing the relationship between the sheet resistanceof the hole transport region and the I_(lead)/I_(oled) ratio of theleakage current I_(leak) flowing from a target organic EL element to anadjacent organic EL element to the current I_(oled) flowing into thetarget organic EL element.

FIG. 3 is a schematic diagram of a display device according to anembodiment of the present disclosure.

FIG. 4 is a plot showing the relationship between the hole mobility ofthe hole transport layer and the I_(leak)/I_(oled) ratio of the leakagecurrent I_(leak) flowing from a target organic EL element to an adjacentorganic EL element to the current I_(oled) flowing into the targetorganic EL element.

FIG. 5 is a plot showing the relationship between the thickness of thefirst luminescent layer and the I_(leak)/I_(oled) ratio of the leakagecurrent I_(leak) flowing from a target organic EL element to an adjacentorganic EL element to the current I_(oled) flowing into the targetorganic EL element.

FIG. 6 is a schematic illustrative representation of a display deviceaccording to an embodiment of the present disclosure.

FIG. 7 is a schematic view of an imaging apparatus according to anembodiment of the present disclosure.

FIG. 8 is a schematic view of a mobile apparatus according to anembodiment of the present disclosure.

FIG. 9A is a schematic view of a display device according to anembodiment of the present disclosure, and FIG. 9B is a schematic view ofa foldable display device according to an embodiment of the presentdisclosure.

FIG. 10 is a schematic representation of a lighting device according toan embodiment of the present disclosure.

FIG. 11 is a schematic representation of a movable body including alighting device according to an embodiment of the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

In the light-emitting device according to an embodiment of the presentdisclosure, leakage current between neighboring organic EL elements isreduced, and the power consumption of the light-emitting device isreduced. Each organic EL element includes a reflection electrode, a holetransport region, an electron-trapping luminescent layer, and a lightextraction electrode. In the embodiments of the present disclosure, thesheet resistance per pixel in the in-plane direction of the holetransport region is 4.0×10⁷ Ω/sq. or more and, thus, the leakage currentbetween organic EL elements is reduced. Also, the presence of theelectron-trapping luminescent layer makes the distance between thereflection electrode and the emission point an optical path length thatenables the emission from the luminescent layer to be enhanced.

FIG. 1 is a schematic sectional view of a light-emitting deviceaccording to an embodiment of the present disclosure. The light-emittingdevice shown in FIG. 1 includes a substrate 1 and three types (R, G, andB) of organic EL elements 10R, 10G, and 10B, that are disposed on thesubstrate and that each include a reflection electrode 2, and an organiccompound layer 4, a light extraction electrode 5, a sealing layer 6, andany one of color filters 7R, 7G, and 7B. The organic compound layer 4and the light extraction electrode 5 are each disposed continuously inthe in-plane direction along the main surface of the substrate 1. Thethree neighboring organic EL elements 10R, 10G, and 10B are separatedfrom each other by an insulating layer 3. More specifically, theinsulating layer 3 is disposed at the end of each reflection electrode 2and disposed at upper surface of the end. The insulating layer 3 isintended to ensure insulation between any two adjacent reflectionelectrodes 2B, 2G, and 2R and insulation between each reflectionelectrode and the light extraction electrode 5 and to accurately definethe light-emitting regions in a desired shape.

The organic compound layer 4 is a common layer shared by the pluralityof organic EL elements. The term “common layer” implies that the layeris disposed across the plurality of organic EL elements and may beformed by a coating process, such as spin coating, or vapor depositionfor the entire surface of the substrate. Fact that the organic compoundlayer 4 is a common layer implies that a plurality of reflectionelectrodes are provided on one organic compound layer.

In the present embodiment, the reflection electrode 2 is positive andthe light extraction electrode 5 is negative.

In the present embodiment, the organic compound layer 4 includes aplurality of layers: a hole transport layer 41, an electron blockinglayer 42, a first luminescent layer 43, a second luminescent layer 44,and an electron transport layer 45. The organic compound layer mayinclude further organic compound layers. The organic compound layersdisposed between the reflection electrode 2 and the first luminescentlayer 43 are collectively referred to as a hole transport region. Inanother embodiment, the hole transport region may include a holeinjection layer, a hole transport layer, and an electron blocking layer.

Each organic EL element is electrically connected to the adjacentorganic EL elements by the hole transport region. In the light-emittingdevice of the present embodiment, the sheet resistance per pixel in thein-plane direction of the hole transport region is 4.0×10⁷ Ω/sq. or moreat a current of 0.1 nA/pixel from the viewpoint of reducing leakagecurrent between neighboring organic EL elements. More likely, the sheetresistance may be 6.0×10⁷ Ω/sq. or more.

FIG. 2 is a plot showing the relationship between the sheet resistanceof the hole transport region and the I_(leak)/I_(oled) ratio of theleakage current I_(leak) flowing from a target organic EL element to anadjacent organic EL element to the current I_(oled) flowing into thetarget organic EL element. Measurement of current I_(oled) and leakagecurrent I_(leak) at a Red organic EL element (R pixel) will be describedby way of example. The Red organic EL element is electrified in a statewhere the adjacent Green and Blue organic EL elements areshort-circuited (potential: 0 V). At this time, the current flowing fromthe reflection electrode of the Red organic EL element to the lightextraction electrode of the Red organic EL element is defined asI_(oled), and the current flowing from the reflection electrode of theRed organic EL element to the reflection electrode of the Green organicEL element (G pixel) or the Blue organic EL element (B pixel) is definedas I_(leak). The sheet resistance per pixel in the in-plane direction iscalculated by using equation (1). In this instance, the sheet resistanceis the value when current I_(oled) is 0.1 nA/pixel.

Rs=dI _(leak) /dV*W/L  (1)

In this equation, W represents the total width of the two adjacentorganic EL elements, L represents the distance between the two adjacentorganic EL elements, and V represents the voltage applied to the targetorganic EL element. dI_(leak)/dV represents the differential resistance.

The hole transport region includes a plurality of organic compoundlayers. The hole transport region may include a hole injection layer, ahole transport layer, and an electron blocking layer. These layers maybe used independently or in combination. Each layer of the holetransport region may be composed of a single compound or may contain aplurality of compounds.

In an embodiment in which the hole transport region includes a holetransport layer, the thickness of the hole transport layer may be lessthan 10 nm. As the thickness of the hole transport layer is smaller, thethickness of the hole transport layer at the side wall of the insulatinglayer decreases. Accordingly, the resistance between the organic ELelements tends to increase. In some embodiments, the thickness of thehole transport layer may be 7 nm or less or 5 nm or less.

In view of the resistance in the in-plane direction, the hole mobilityof the hole transport layer in the direction parallel to the mainsurface of the substrate is beneficially 2.5×10⁻³ cm²/(V·s) or less, forexample, 1.0×10⁻³ cm²/(V·s) or less.

FIG. 4 is a plot showing the relationship between the hole mobility ofthe hole transport layer and the I_(leak)/I_(oled) ratio of the leakagecurrent I_(leak) flowing from a target organic EL element to an adjacentorganic EL element to the current I_(oled) flowing into the targetorganic EL element.

In an embodiment in which the hole transport region further includes anelectron blocking layer, it is beneficial that the differences inionization potential at the interface between the hole transport layerand the electron blocking layer and at the interface between theelectron blocking layer and the first luminescent layer are each smallfrom the viewpoint of reducing hole accumulation at the interfaces.

It is also beneficial that the layers from the reflection electrode tothe first luminescent layer have a stepwise energy structure in whichthe ionization potential increases step by step from the reflectionelectrode to the first luminescent layer. More specifically, it isbeneficial that the ionization potential of the hole transport layer liebetween the work function of the reflection electrode and the ionizationpotential of the first luminescent layer. It is also beneficial that theionization potential of the electron blocking layer lie between theionization potential of the hole transport layer and the ionizationpotential of the first luminescent layer.

The ionization potentials of an organic compound layer containing aplurality of compounds can be estimated by photoelectron yieldspectroscopy, photoelectron spectroscopy, or the like.

The properties such as hole mobility and ionization potential of thehole transport layer may be controlled by forming the hole transportlayer of a mixture containing two or more hole transporting materials.Thus, the effective distance of the hopping site is increased and,consequently, the resistance at the side wall of the insulating layer isfurther increased. By adding a compound of the electron blocking layerinto the hole transport layer, the ionization potential of the holetransport layer approaches the ionization potential of the electronblocking layer and, thus, hole accumulation at the interface between thehole transport layer and the electron blocking layer is reduced. If thecompound used in the electron blocking layer has a high resistivity, theresistivity of the hole transport layer increases, and this isbeneficial.

For example, the hole transport layer may contain a first compound and asecond compound. In this instance, the hole mobility of the firstcompound may be 1.0×10⁻³ cm²/(V·s) or less, and, in some embodiments, itmay be 5.0×10⁻⁴ cm²/(V·s) or less.

The highest occupied molecular orbital (HOMO) of the first compound maybe lower than the HOMO of the second compound. The HOMO of the firstcompound may be 0.1 eV or more lower than the HOMO of the secondcompound.

The percentage of the weight of the first compound with respect to thetotal weight of the first compound and the second compound may be in therange of 50% to 95%, for example, 75% to 95%.

In an embodiment in which the hole transport region includes a holeinjection layer, the hole injection layer is an organic compound layerdisposed between the reflection electrode and the hole transport layer.The hole injection layer may contain a compound having an electronaffinity of 5.0 eV or more. The thickness of the hole injection layermay be 10 nm or less from the viewpoint of reducing leakage currentbetween the neighboring organic EL elements. The organic compound usedin the hole injection layer may have a lowest unoccupied molecularorbital (LUMO) of −5.0 eV or less.

Examples of the compound having an electron affinity of 5.0 eV or moreinclude hexaazatriphenylene derivatives and tetracyanoquinodimethanederivatives. In some embodiments, a hexacyano hexaazatriphenylenecompound may be used.

The organic EL element includes at least one luminescent layer. If twoor more luminescent layers are included, the two layers (a first and asecond luminescent layer) may be separated by a further organic compoundlayer.

The first and the second luminescent layer may each emit light havingany wavelength. For example, the first luminescent layer may emit bluelight, and the second luminescent layer may emit green light and redlight. Thus, the organic EL element may emit white light. Alternatively,the first luminescent layer may emit green light and red light for anorganic EL element operable to emit white light. The emission color ofthe first luminescent layer and the emission color of the secondluminescent layer may be complementary to each other for emitting whitelight. In some embodiments, the first luminescent layer does not emitblue light: hence, the first luminescent layer emits light other thanblue light. If a luminescent layer operable to emit blue light is closeto a metal electrode, surface plasmon loss increases, and the powerconsumption of the device increases accordingly. A luminescent layeroperable to emit blue light implies that the luminescent layer containsa luminescent material capable of emitting blue light.

From the viewpoint of reducing the power consumption of thelight-emitting device, the luminous efficiency of the light-emittingdevice may be increased by optical interference.

In the present disclosure, the luminescent layer is of an electrontrapping type so as to maintain the optical path length enabling theemission from the luminescent layer to be enhanced even though thethickness of the hole transport region is reduced. Since theelectrode-trapping luminescent layer emits light mainly on the sidetoward the negative electrode, the distance from the emission point tothe reflection electrode is the sum of the thickness of the holetransport region and the thickness of the luminescent layer. Therefore,even if the thickness of the hole transport region is reduced, thethickness of the luminescent layer compensates for the reduction, thusmaintaining the distance for optical interference.

Thus, the first luminescent layer is an electron-trapping luminescentlayer. The electron-trapping luminescent layer mentioned herein containsa first compound and a second compound, and the compound having a higherweight has a lower electron affinity than the other. The electronaffinity of a compound may be estimated by the lowest unoccupiedmolecular orbital (LUMO) level of the molecule of the compound. When theelectron affinities are estimated by LUMO, one of the compounds having ahigher weight than the other in the electron-trapping luminescent layerhas a higher LUMO level than the other. A higher LUMO level is closer tothe vacuum level, and a high LUMO level may be referred to as a shallowLUMO level or a small absolute value of LUMO level. In general, electronaffinity is represented by an absolute value, and LUMO level isrepresented by a real number. More specifically, electron affinity isrepresented by a positive number, and LUMO level is represented by anegative number.

For the electron-trapping luminescent layer, the first compound may beselected from among pyrene derivatives, anthracene derivatives, fluorenederivatives, and naphthalene derivatives. If the first compound accountsfor the largest part of the weight of the luminescent layer, the firstcompound is referred to as the host material or the host.

The second compound may be selected from among pyrene derivatives,fluoranthene derivatives, fluorene derivatives, and chrysenederivatives. A derivative of the first compound or the second compoundrefers to a form whose base skeleton has a substituent or a condensedring. For example, fluoranthene derivatives include benzofluoranthene,dibenzofluoranthene, and indenobenzo[k]fluoranthene. If the firstcompound is the host, the second compound is referred to as a dopant ora guest.

The substituents of derivatives may include alkyl groups, aryl groupshaving a carbon number of 6 to 60, and heteroaryl groups having a carbonnumber of 6 to 60.

The distance between the first luminescent layer and the reflectionelectrode may satisfy the following relationship (2) from the viewpointof enhancing the emission from the first luminescent layer. The smallerthe thickness of the first luminescent layer, the better. However, evenif the thickness of the first luminescent layer is large, leakagecurrent in the light-emitting device can be reduced because theinfluence of the luminescent layer on leakage current is small.

(0.12−(φ_(r)/4π))<L/λ ₁<(0.18−((φ_(r)/4π))  (2)

Here λ₁ represents the shortest of the wavelengths at which an emissionspectrum of the first luminescent layer has peaks, and φ_(r) representsthe phase shift at the reflection electrode.

When relationship (2) holds true, surface plasmon loss is reduced toreduce the power consumption of the light-emitting device.

In the embodiment in which the hole transport region includes a holetransport layer and an electron blocking layer, the first luminescentlayer may satisfy the following relationship (3):

d _((1st-EML)) >d _((HTL)) +d _((EBL))  (3),

wherein d_((1st-EML)) represents the thickness of the first luminescentlayer, and d_((HTL)) and d_((EBL)) represent the thickness of the holetransport layer and the thickness of the electron blocking layer,respectively.

The thickness of the first luminescent layer may be 35 nm or less. Whenthe thickness of the first luminescent layer is 35 nm or less, leakagecurrent between neighboring organic EL elements is further reduced. Thiswill be verified herein later.

In an embodiment, the organic EL element may further include an electrontransport layer between the luminescent layer and the light extractionelectrode. The material of the electron transport layer is selected inview of the balance with the hole mobility of the hole transport layer.The material of the electron transport layer may be selected from agroup including aromatic hydrocarbons such as chrysene derivatives,fluoranthene derivatives, and anthracene derivatives; heterocycliccompounds such as phenanthroline derivatives, diazafluoranthenederivatives, and azaanthracene derivatives; and organic metal complexessuch as tris(8-hydroxyquinolinato)aluminum (Alq3), beryllium complexes,and magnesium complexes.

The R, G, and B sub pixels are arranged in a stripe array, a squarearray, a delta array, or a Bayer array.

In some embodiments, the reflection electrode of the organic EL elementmay be made of a metal material having a reflectance of 80% or more.More specifically, the material of the reflection electrode may be ametal, such as Al or Ag, or an alloy thereof with Si, Cu, Ni, Nd, Ti, orthe like. The term reflectance mentioned here refers to the reflectanceat the emission wavelength of the luminescent layer. The reflectionelectrode may include a barrier layer on the side toward the lightextraction electrode. The barrier layer may be made of a metal, such asTi, W, Mo, or Au, or an alloy thereof.

The insulating layer of the organic EL element of the present disclosuremay be formed of silicon nitride (SiN), silicon nitroxide (SiON), orsilicon oxide (SiO) by chemical vapor deposition (CVD).

From the viewpoint of increasing the resistance of the organic compoundlayer in the region between organic EL elements, the thickness of theorganic compound layer at the side wall of the insulating layer may besmaller than that in the aperture region. There are emission points ofthe organic EL element in the aperture region. More specifically, thethickness of the organic compound layer at the side wall may be reducedby increasing the angle between the substrate and the side wall of theinsulating layer or increasing the thickness of the insulating layer.The angle between the substrate and the side wall of the insulatinglayer may be referred to as taper angle of the insulating layer.

The angle between the substrate and the side wall of the insulatinglayer may be in the range of 60 degrees to 90 degrees. Also, thethickness of the insulating layer may be in the range of 40 nm to 150nm.

The insulating layer may have a groove between the reflection electrodeand the adjacent reflection electrode. The presence of the groove helpsreduce the thickness of the organic compound layer and increase theresistance.

In an embodiment, the light extraction electrode of the organic ELelement may be a semi-transmissive reflection layer that transmits aportion of the light incident on the surface thereof and reflects theother portion (that is, has transmission and reflectioncharacteristics). The light extraction electrode is made of, forexample, an elemental metal, such as magnesium or silver, or an alloycontaining mainly magnesium or silver or containing an alkali metal oran alkaline-earth metal. The light extraction electrode may have amultilayer structure provided that it has a favorable transmittance.

The sealing layer of the organic EL element may be formed by chemicalvapor deposition (CVD) or atomic layer deposition (ALD). The sealinglayer may be made of a material having a very low permeability toexternal oxygen and moisture, such as silicon nitride (SiN), siliconnitroxide (SiON), aluminum oxide, silicon oxide, or titanium oxide. Thesealing layer may be a single-layer or a multilayer structure providedthat it can sufficiently block moisture. If the sealing layer has amultilayer structure, it may be defined by a combination of a SiN layerand an aluminum oxide layer. The multilayer structure may include threeor more layers.

In an embodiment, the hole transport region of the organic EL elementmay contain any of the following compounds HT1 to HT38:

The compound used in the hole transport region may be represented by oneof the following general formulas [1] and [2]:

wherein Ar₁ to Ar₃ each represent one independently selected from thegroup consisting of substituted or unsubstituted aryl groups includingphenyl, bisphenyl, terphenyl, fluorenyl, naphthyl, and spirofluorenyland substituted or unsubstituted heterocyclic groups includingdibenzofuranyl, dibenzothiophenyl, thiophenyl, furanyl, and carbazolyl;and

wherein Ar₄ represents a substituted or unsubstituted aryl groupselected from the group consisting of phenyl, biphenyl, terphenyl,fluorenyl, dibenzofuranyl, dibenzothiophenyl, and naphthyl, and Ar₅ toAr₈ each represent one independently selected from the group consistingof substituted or unsubstituted aryl groups including phenyl, biphenyl,terphenyl, fluorenyl, phenanthrenyl, and pyrenyl and substituted orunsubstituted heterocyclic groups including dibenzofuranyl,dibenzothiophenyl, thiophenyl, furanyl, and carbazolyl.

The light-emitting device may be used in a display device including anactive element, such as a transistor. The display device includes alateral driving circuit, a vertical driving circuit, and a displaysection, and the display section includes the light-emitting deviceaccording to an embodiment of the present disclosure.

FIG. 3 is a schematic diagram of a display device according to anembodiment of the present disclosure. The display device 15 includes adisplay region 11, a lateral driving circuit 12, a vertical drivingcircuit 13, and a connecting section 14. The display region 11 may havethe light-emitting device according to an embodiment of the presentdiscloser.

In an embodiment, the display device may be used as a display section ofan image forming apparatus, such as a multifunctional printer or an inkjet printer. In this instance, the display section may have both adisplaying function and an operational function.

In an embodiment, the display device may be used as a display section ofan imaging apparatus, such as a camera, including an optical systemhaving a plurality of lenses and an imaging element capable of receivinglight that has passed through the optical system. The display section ofthe imaging apparatus may be used to display information obtained by theimaging element. The display section may be exposed to the outside ofthe imaging apparatus or may be disposed within a viewfinder.

In an embodiment, the display device may include a red, a green, and ablue color filter. The red, green, and blue color filters may bearranged in a delta array.

In an embodiment, the display device of the present disclosure may beused in the display section of a mobile terminal. In this instance, thedisplay section may have both a displaying function and an operationalfunction.

An organic EL element according to an embodiment will now be described.

The organic EL element includes a pair of electrodes (anode and cathode)and an organic compound layer between the electrodes. The organiccompound layer may be composed of a single layer or have a multilayerstructure including a plurality of layers, provided that the organiccompound layer includes a luminescent layer.

The luminescent layer may contain a host and a guest. The luminescentlayer may also contain an assist. The host mentioned here refers to acompound accounting for the highest percentage of the total weight ofthe compounds in the luminescent layer. The guest mentioned here refersto a compound in the luminescent layer having a lower weight than thehost and is responsible for main emission. The assist material mentionedhere refers to a compound in the luminescent layer having a lower weightthan the host and helps the guest emit light. The assist material may bereferred to as a second host. If the luminescent layer of the organicluminescent element has a uniform composition throughout the layer, thecomposition of the entire luminescent layer can be determined byanalyzing a portion of the luminescent layer.

When the organic compound disclosed herein is used as the guest of theluminescent layer, the guest content may be in the range of 0.01% byweight to 20% by weight, for example, 0.1% by weight to 5% by weight,relative to the total weight of the luminescent layer.

Also, when the luminescent layer contains a host and a guest, the hostmay be a compound having a higher LUMO level than the guest. This isbecause the electron trap type guest has a low LUMO, and by using acompound having a higher LUMO than the organic compound of the presentdisclosure as the host, the organic compound of the present disclosurecan receive a larger part of the electrons applied to the host.

The luminescent layer may be composed of a single layer or may have amultilayer structure. Also, the luminescent layer may contain anotherluminescent material having another emission color to emit a color lightmixed with red that is the emission color of the present disclosure. Themultilayer structure refers to a state where different luminescentlayers are formed one on top of another. In this instance, the emissioncolor of the organic luminescent element is not limited to red. Forexample, the emission color may be white or intermediate color. If theemission color is white, the additional luminescent layer emits a colorlight other than red light, such as blue or green. The luminescent layermay be formed by vapor deposition or coating. The organic compound ofthe present disclosure may be used in other organic compound layers ofthe organic luminescent element as well as in the luminescent layer. Forexample, the electron transport layer, the electron injection layer, thehole transport layer, the hole injection layer, the hole blocking layer,or any other layer may contain the organic compound of the presentdisclosure. In this instance, the emission color of the organicluminescent element is not limited to red. For example, the emissioncolor may be white or intermediate color.

The organic compound may be used in combination with one or more knownlow-molecular-weight or polymeric compounds used as a hole injecting ora hole transporting material, a host, a luminescent material, anelectron injecting or an electron transporting material, and the like,if necessary.

These compounds are as follows. The hole injecting or transportingmaterial may have so high a hole mobility as facilitates hole injectionfrom the anode and as enables the injected holes to be transported tothe luminescent layer. Also, from the viewpoint of reducing thecrystallization or any other deterioration of the material in theorganic luminescent element, the hole injecting or transporting materialmay have a high glass transition temperature. Low-molecular-weight orpolymeric hole injecting or transporting materials include triarylaminederivatives, arylcarbazole derivatives, phenylenediamine derivatives,stilbene derivatives, phthalocyanine derivatives, porphyrin derivatives,poly(vinyl carbazole), polythiophene, and other electrically conductivepolymers. The hole injecting or transporting material may also be usedin the electron blocking layer.

The electron transporting material may be a compound capable oftransporting electrons injected from the cathode to the luminescentlayer and may be selected in view of the balance with the hole mobilityof the hole transporting material. Electron transporting materialsinclude oxadiazole derivatives, oxazole derivatives, pyrazinederivatives, triazole derivatives, triazine derivatives, quinolinederivatives, quinoxaline derivatives, phenanthroline derivatives,organic aluminum complexes, and condensed ring compounds (such asfluorene derivatives, naphthalene derivatives, chrysene derivatives, andanthracene derivatives). The electron transporting material may also beused in the hole blocking layer.

FIG. 6 is a schematic illustrative representation of a display deviceaccording to an embodiment of the present disclosure. The display device1000 may include a touch panel 1003, a display panel 1005, a frame 1006,a circuit board 1007, and a battery 1008 between an upper cover 1001 anda lower cover 1009. The touch panel 1003 and the display panel 1005 areconnected to a flexible printed circuits (FPCs) 1002 and 1004,respectively. Transistors are printed on the circuit board 1007. Thebattery 1008 is not necessarily provided unless the display device isfor mobile use, and the position of the battery is not limited to theposition shown in the figure even for mobile use.

In an embodiment, the display device of the present disclosure may beused as a display section of an imaging apparatus including an opticalsystem having a plurality of lenses and an imaging element capable ofreceiving light that has passed through the optical system. The displaysection of the imaging apparatus may be used to display informationobtained by the imaging element. The display section may be exposed tothe outside of the imaging apparatus or may be disposed within aviewfinder. The imaging apparatus may be a digital camera or a digitalvideo camera.

FIG. 7 is a schematic view of an imaging apparatus according to anembodiment of the present disclosure. The imaging apparatus 1100 mayinclude a viewfinder 1101, a rear display 1102, an operational section1103, and a housing 1104. The viewfinder 1101 may include the displaydevice according to an embodiment of the present disclosure. In thisinstance, the display device may display not only taken images but alsoenvironmental information, imaging instructions, or the like. Theenvironmental information may include, for example, the intensity andthe direction of external light, the moving speed of a subject to betaken, and the possibility that the subject is hidden by an object.

Since the appropriate timing for taking an image is a very short periodof time, it is desirable to display information as quickly as possible.Accordingly, the display device using organic EL elements according toan embodiment of the present disclosure is useful. This is becauseorganic EL elements respond quickly. The display device using organic ELelements is more suitable than liquid crystal display devices for use inapparatuses required to display information quickly.

The imaging apparatus 1100 includes an optical system (not shown). Theoptical system includes a plurality of lenses and forms an image on theimaging element in the housing 1103. The focus can be adjusted byadjusting the relative positions of the plurality of lenses. This may beautomatically performed.

In an embodiment, the display device may include a red, a green, and ablue color filter. The red, green, and blue color filters may bearranged in a delta array.

In an embodiment, the display device of the present disclosure may beused in the display section of a mobile terminal. In this instance, thedisplay section may have both a displaying function and an operationalfunction. The mobile terminal may be a cellular phone, such as asmartphone, a tablet PC, a head-mounted display, or the like.

FIG. 8 is a schematic view of an electric apparatus according to anembodiment of the present disclosure. The mobile apparatus 1200 includesa display section 1201, an operational section 1202, and a housing 1203.The housing 1203 contains a circuit, a printed board having the circuit,a battery, and a communication section. The operational section 1202 maybe a button or a touch panel responder. The operational section 1202 mayhave a biometrically authenticating function of recognizing thefingerprint and releasing the lock. The mobile apparatus including acommunication section may be referred to as a communication apparatus.

FIGS. 9A and 9B are schematic illustrative representations of displaydevices each according to an embodiment of the present disclosure. FIG.9A shows a display device used as a TV monitor or a PC monitor. Thisdisplay device 1300 includes a frame 1301 and a display section 1302.The display section 1302 may include the light-emitting device accordingto an embodiment of the present disclosure.

The display device also includes a base 1303 supporting the frame 1301and the display section 1302. The base 1303 is not limited to the formshown in FIG. 9A. Alternatively, the lower side of the frame 1301 mayserve as the base.

The frame 1301 and the display section 1302 may be curved. The radius ofcurvature thereof may be in the range of 5000 mm to 6000 mm.

FIG. 9B is a schematic illustrative representation of a display deviceaccording to another embodiment of the present disclosure. The displaydevice 1310 shown in FIG. 9B is a foldable display device. The displaydevice 1310 includes a first display section 1311, a second displaysection 1312, and a housing 1313 and has a folding line 1314. The firstdisplay section 1311 and the second display section 1312 each mayinclude the light-emitting device according to an embodiment of thepresent disclosure. The first display section 1311 and the seconddisplay section 1312 may be continuous without being separated by ajoint. The first display section 1311 and the second display section1312 may be separated from each other along the folding line 1314. Thefirst display section 1311 and the second display section 1312 maydisplay different images from each other, or a single image may bedisplayed on a set of the first and second display sections.

FIG. 10 is a schematic illustrative representation of a lighting deviceaccording to an embodiment of the present disclosure. The lightingdevice 1400 may include a housing 1401, a light source 1402, a circuitboard 1403, an optical filter 1404, and a light diffusing section 1405.The light source 1402 may include the organic EL element according to anembodiment of the present disclosure. The optical filter 1404 may beintended to improve the color rendering properties of the light source1402. The light diffusion section 1405 diffuses light emitted from thelight source 1402 effectively and helps the light reach a wide regionfor, for example, lighting up. A cover may be provided at an outermostportion.

The region of emission of the lighting device 1400 may be separated fromeach other. The light-emitting device of the present disclosure iseffective in suppressing emission from an undesired region.

The lighting device illuminates, for example, a room. The lightingdevice may emit light of cool white, sunshine color, or any other colorfrom blue to red. The lighting device may include a dimmer circuit thatdims the light. The lighting device may include the organic luminescentelement according to an embodiment of the present disclosure and a powersupply circuit connected to the organic luminescent element. The powersupply circuit converts alternating voltage to direct voltage. White hasa color temperature of 4200 K and sunshine color has a color temperatureof 5000 K. The lighting device may include a color filter.

The lighting device may include a heat radiation section. The heatradiation section is intended to dissipate heat from the device and maybe made of, for example, a metal having a high specific heat or liquidsilicon.

FIG. 11 is a schematic view of an automobile that is an implementationof the movable body according to an embodiment of the presentdisclosure. The automobile 1500 has a tail lamp 1501 that is a type oflighting device, and the tail lamp 1501 may light when the breaks areapplied.

The tail lamp 1501 may include the organic luminescent element accordingto an embodiment of the present disclosure. The tail lamp may include aprotective member that protects the organic EL element. The protectivemember may be made of any material provided that it has a strength tosome extent and is transparent. In some embodiments, the protectivemember may be made of polycarbonate or the like. The polycarbonate maybe mixed with a furandicarboxylic acid derivative, an acrylonitrilederivative, or the like.

The automobile 1500 may include a car body 1503 and a window 1502attached to the car body 1503. The window 1502 may be a transparentdisplay unless it is intended for checking of the front and rear of theautomobile. The transparent display may include the organic luminescentelement according to an embodiment of the present disclosure. In thisinstance, the members such as electrodes of the organic luminescentelement are made of a transparent material.

In an embodiment, the movable body may be a ship, an aircraft, a drone,or the like. The movable body may include an enclosure and a lightingdevice provided for the enclosure. The lighting device may emit light toprovide a notification of the position of the enclosure. The lightingdevice includes the organic luminescent element according to anembodiment of the present disclosure.

In an embodiment, the organic luminescent element may be used fordisplaying an image. In this instance, the emission from the organicluminescent element has a luminance that is controlled by a TFT, orswitching element, and a plurality of such organic EL elements arearranged in a plane so that an image is displayed by emission luminancesof the organic luminescent elements. The TFT may be substituted by anyother switching element, such as a transistor made of a low-temperaturepolysilicon or an active matrix driver on or in a substrate, such as asilicon substrate. Whether on a substrate or in a substrate depends ondefinition. For example, for a definition of a QVGA level for 1 inch,the organic EL elements may be disposed on a silicon substrate. Thedisplay device including the organic luminescent elements according tothe present disclosure is operable to display high-quality images over along time.

EXAMPLES Example 1

Light-emitting device D100 was produced by the process described below,and the resulting light-emitting device was examined for leakage betweenneighboring organic EL elements, the power consumption, and otherproperties of the device.

As shown in FIG. 1, reflection electrodes 2 were formed on a substrateby patterning, and an insulating layer 3 was formed between the organicEL elements. The insulating layer 3 was a silicon oxide film and had athickness of 65 nm. The taper angle between the side wall and thesubstrate was 80° at the aperture for the organic EL elements and 75° atthe region between the organic EL elements. The pixels were arranged ina delta array with a distance of 1.4 μm between apertures and a distanceof 0.6 μm between the reflection electrodes. The hole injection layerwas formed of the following compound 1 to a thickness of 3 nm over thereflection electrodes.

The hole transport layer was formed of exemplified compound HT9 to athickness of 15 nm, and an electron blocking layer was formed ofexemplified compound HT27 to a thickness of 10 nm. A first luminescentlayer containing 97% by weight of compound 2 shown below as the hostmaterial and 3% by weight of compound 3 shown below as the emissiondopant was formed to a thickness of 10 nm. The hole mobilities ofexemplified compounds HT9 and HT27 and compound 2 were 2×10⁻³ cm²/(V·s),5×10⁻⁴ cm²/(V·s), and 1×10⁻³ cm²/(V·s), respectively.

A second luminescent layer containing 99% by weight of compound 2 as thehost material and 1% by weight of compound 4 shown below as the emissiondopant was formed.

An electron transport layer was formed of compound 5 shown below to athickness of 110 nm. An electron injection layer was formed of LiF to athickness of 0.5 nm. The light extraction electrode was formed of a MgAgalloy to a thickness of 10 nm. The ratio of Mg to Ag was 1:1. Then, SiNwas deposited to a thickness of 1.5 μm by CVD to yield a sealing layer.

Table 1 shows the specifications used for estimating the powerconsumption of the display device of the present Example. The organic ELelement aperture ratio was 50%, and the R, G, and B organic EL elementaperture ratios were each 16.7%. For the estimation of the powerconsumption, the power required for the display device with thespecification shown in Table 1 to emit white light having a colortemperature of 6500 K (CIE(x,y)=(0.313, 0.329)) and a luminance of 500cd/cm² was calculated. More specifically, the current required for theR, the G, and the B organic EL elements was calculated from the measuredluminous efficiency. The power consumption was calculated from therequired current value on the assumption that the driving voltage was10.0 V.

TABLE 1 Unit Diagonal inch 0.5 [inch] Vertical proportion 3 Lateralproportion 4 Sub pixel aperture ratio 16.7 [%] Pixel aperture ratio 50[%] Intended chromaticity CIE_x 0.313 CIE_y 0.329 White light luminance500 [cd/m²] Driving voltage (Fixed) 10 [V]

The leakage between organic EL elements was estimation by using as anindex the I_(leak)/I_(oled) ratio of current I_(leak) flowing betweenthe pixels to current I_(oled) flowing in each organic EL element whencurrent I_(oled) was 0.1 [nA/pixel].

Similarly, light-emitting devices D101 to D108 were produced in the samemanner as device D100, except that each layer was formed to thethickness shown in Table 2. Table 2 shows the thicknesses of the layersof light-emitting devices D100 to D108 and measurement results.

TABLE 2 Hole Hole Electron First injection transport blockingluminescent Power layer layer layer layer consumption Element [nm] [nm][nm] [nm] I_(leak)/I_(oled) [mW] D100 Comparative 3 15 10 10 0.55 Bad357 Good Example D101 Comparative 3 10 10 10 0.42 Bad 409 Bad ExampleD102 Comparative 3 5 10 10 0.21 Good 488 Bad Example D103 Comparative 315 10 15 0.61 Bad 324 Good Example D104 Comparative 3 10 10 15 0.48 Bad363 Good Example D105 Comparative 3 5 10 15 0.24 Good 422 Bad ExampleD106 Comparative 3 15 10 20 0.67 Bad 313 Good Example D107 Comparative 310 10 20 0.56 Bad 332 Good Example D108 Example 3 5 10 20 0.26 Good 376Good

The devices exhibiting an I_(leak)/I_(oled) ratio of 0.35 or less weredetermined to be good, and the devices exhibiting an I_(leak)/I_(oled)ratio of more than 0.35 were determined to be bad. For powerconsumption, the devices of 400 mW or less were determined to be good,and the devices of more than 400 mW were determined to be bad.

Example 2

Light-emitting devices D109 to D114 used in the present Example were thesame as device D108 except that the hole transport layer was anintermixed layer of exemplified compounds HT37 and HT27. Table 3 showsthe ratio of the compounds in the hole transport layer and the resultsof hole mobility measurement. By using an intermixed layer as the holetransport layer, the hole mobility can be reduced, and by increasing theproportion of the compound having a low hole mobility, that is, compoundHT27, the resistance in the in-plane direction can be dramaticallyincreased.

TABLE 3 Mixing ratio of hole transport layer compound to electrontransport Ele- layer compound Hole ment (HT37:HT27) mobility D109 —100:0  4.6 × 10⁻³ D110 Comparative 85:15 3.9 × 10⁻³ Example D111Comparative 70:30 3.2 × 10⁻³ Example D112 Example 50:50 2.3 × 10⁻³ D113Example 25:75 1.2 × 10⁻³ D114 Example 15:85 4.6 × 10⁻⁴

FIG. 4 is a plot of the relationship between the hole mobility and theI_(leak)/I_(oled) ratio of leakage between pixels. Since the thicknessesof the devices shown in Table 3 are the same, the power consumptions ofthe devices depending on optical interference are the same. FIG. 4 showsthat as the hole mobility of the hole transport layer is increased, theratio of leakage between pixels increases. In particular, when the holemobility exceeds 2.5×10⁻³ cm²/(V·s), the gradient increases.

These results suggest that the beneficial hole mobility is 2.5×10⁻³cm²/(V·s) or less. If the hole mobility of the hole transport layer isas low as that of device D114, the thickness of the hole transportregion may be increased. However, an excessively low mobility leads toan increased driving voltage. This should be considered for increasingthe thickness. The thickness of the hole transport layer may be 10 nm orless.

Example 3

In the present Example, device D112 was compared with devices D115 andD116. As shown in Table 4, devices D115 and D116 have the same structureas device D112 except for the structure of the hole transport layer.

It is known that an energy barrier is reduced to reduce holeaccumulation by providing an intermixed layer of the hole transportlayer and the electron blocking layer between the hole transport layerand the electron blocking layer.

A decrease of hole accumulation may cause a decrease in resistance inthe in-plane direction. Accordingly, the effect of the intermixed layerwas examined by comparing devices D115 and D116 with device D112. DeviceD115 was produced in the same manner as device D112 except that a holetransport layer containing a hole transporting material HT37 was formedto a thickness of 5 nm and then an intermixed layer of HT37 and HT27 ina ratio of 50:50 was formed to a thickness of 5 nm. Also, device D116was produced in the same manner as device D112 except that a holetransport layer containing a hole transporting material HT37 was formedto a thickness of 5 nm, but an intermixed layer was not formed.

TABLE 4 Thickness [nm] Hole Electron Rate of transport Intermixed holeblocking leakage layer transport layer layer I_(leak)/I_(oled) D112Example 0 5 10 0.24 Good D115 Comparative 5 5 10 1.81 Bad Example D116Comparative 5 0 10 1.32 Bad Example

As shown in Table 4, in device D116 using H37 having a high holemobility, the ratio of leakage between pixels was as high as 1.32. Fordevice D115 provided with an intermixed layer between the hole transportlayer and the electron blocking layer, the ratio of leakage betweenpixels was 1.81.

These results suggest that the resistance to leakage between pixels doesnot depend on whether hole accumulation is reduced by the presence of anintermixed layer, but depends mainly on the total thickness of the holetransport layer and the electron blocking layer.

FIG. 5 is a plot showing the relationship between the thickness of thefirst luminescent layer and the (I_(leak)/I_(oled)) ratio of the leakagecurrent I_(leak) flowing from a target organic EL element to an adjacentorganic EL element to the current I_(oled) flowing into the targetorganic EL element. The threshold of the thickness of the firstluminescent layer at which the I_(leak)/I_(oled) ratio is 0.35 or lessis estimated to be 35 nm or less based on the above examination aboutleakage current.

As described above, the present disclosure provides a light-emittingdevice including organic EL elements in which leakage current betweenneighboring organic EL elements is reduced, and having a low powerconsumption reduced by using optical interference.

While the present disclosure has been described with reference toexemplary embodiments, it is to be understood that the disclosure is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

What is claimed is:
 1. A light-emitting device comprising: a reflectionelectrode: a first luminescent layer being of an electron trapping type;and a light extraction electrode, in this order, wherein the firstluminescent layer includes a first surface and a second surface closerto the light extraction electrode than the first surface, and wherein adistance between the reflection electrode and the second surface of thefirst luminescent layer is equivalent to a length enabling emission fromthe first luminescent layer to be enhanced.
 2. The light-emitting deviceaccording to claim 1, further comprising a hole transport regiondisposed between the reflection electrode and the first luminescentlayer, wherein the hole transport region of the light-emitting deviceand a hole transport layer of anther light-emitting device are disposedcontinuously.
 3. The light-emitting device according to claim 2, whereinthe hole transport region includes a hole transport layer and anelectron blocking layer.
 4. The light-emitting device according to claim2, wherein the hole transport layer contains a first compound and asecond compound whose HOMO is higher than HOMO of the first compound. 5.The light-emitting device according to claim 4, wherein a weight ratioof the first compound with respect to the total weight of the firstcompound and the second compound is equal to or more than that of thesecond compound.
 6. The light-emitting device according to claim 5,wherein the weight ratio of the first compound is range from 50% to 95%.7. The light-emitting device according to claim 3, wherein the holetransport layer contains a first compound and a second compound, and theelectron blocking layer contains the first compound.
 8. Thelight-emitting device according to claim 1, further including a secondluminescent layer between the first luminescent layer and the lightextraction electrode.
 9. The light-emitting device according to claim 2,wherein the hole transport layer includes one of compounds representedby general formulas [1] and [2]:

wherein Ar₁ to Ar₃ each represent one independently selected from thegroup consisting of substituted or unsubstituted aryl groups selectedfrom the group consisting of phenyl, bisphenyl, terphenyl, fluorenyl,naphthyl, and spirofluorenyl and substituted or unsubstitutedheterocyclic groups selected from the group consisting ofdibenzofuranyl, dibenzothiophenyl, thiophenyl, furanyl, and carbazolyl;and

wherein Ar₄ represents a substituted or unsubstituted aryl groupselected from the group consisting of phenyl, biphenyl, terphenyl,fluorenyl, dibenzofuranyl, dibenzothiophenyl, and naphthyl, and Ar₅ toAr₈ each represent one independently selected from the group consistingof substituted or unsubstituted aryl groups consisting of phenyl,biphenyl, terphenyl, fluorenyl, phenanthrenyl, and pyrenyl; andsubstituted or unsubstituted heterocyclic groups includingdibenzofuranyl, dibenzothiophenyl, thiophenyl, furanyl, and carbazolyl.10. The light-emitting device according to claim 9, wherein the holetransport layer contains at least two compounds selected from thecompounds represented by the general formulas [1] and [2].
 11. Thelight-emitting device according to claim 1, wherein the light-emittingdevice further includes a hole injection layer disposed between thereflection electrode and the first luminescent layer, the hole injectionlayer containing a compound having an electron affinity of 5.0 eV ormore.
 12. The light-emitting device according to claim 2, wherein thehole transport layer has a thickness of 5 nm or less.
 13. Thelight-emitting device according to claim 1, the light-emitting devicefurther includes a second luminescent layer between the firstluminescent layer and the light extraction electrode, the firstluminescent layer being operable to emit light other than blue light,the second luminescent layer being operable to emit blue light.
 14. Adisplay device comprising: the light-emitting device as set forth inclaim 1; and an active element connected to the light-emitting device.15. An imaging apparatus comprising: an optical system including aplurality of lenses; an imaging element capable of receiving light thathas passed through the optical system; and a display section on whichinformation obtained by the imaging element is displayed, the displaysection including the light-emitting device as set forth in claim
 1. 16.A movable body comprising: an enclosure; and a lighting device providedfor the enclosure, the lighting device including the light-emittingdevice as set forth in claim 1.